In recent years, because of reductions in sizes of portable electronic devices and development of hybrid automobiles, electric automobiles, etc. due to consideration of environmental problems such as increases in atmospheric pollution and carbon dioxide, secondary batteries having excellent characteristics, such as high efficiency, high output, high energy density, and light weight are needed for such electronic devices and electric automobiles. Various secondary batteries having such characteristics have been developed and researched. Lithium secondary batteries have been also variously researched in order to provide ones having such characteristics.
A chargeable and dischargeable lithium battery usually has a structure in which a space between a positive electrode (cathode) and a negative electrode (anode) is filled with a lithium salt, such as LiPF6, dissolved in an electrolytic solution, for example, a non-aqueous organic solvent. Lithium transition metal oxide is used as the positive electrode, and lithium or carbon (graphite) is mainly used as the negative electrode. The electrolytic solution has a good ionic conductivity and a negligible electrical conductivity. During charging, lithium ions move from the positive electrode to the negative electrode, and during discharging, lithium ions move in the reverse direction.
The positive electrode and the negative electrode of the lithium battery are separated from each other with a separator of a porous polymer film and are formed into a structure preventing their direct electric contact. Accordingly, the separator for a secondary battery is required to have various characteristics, such as film thickness (thinness), mechanical strength, ionic conductance (during containing of an electrolytic solution), electric insulation, electrolytic solution resistance, electrolytic solution-retaining property, and wettability. As the separators for secondary batteries having these properties, fine porous films made of polyolefins, such as polyethylene and polypropylene, are generally used. These fine porous films have random pores at a porosity of about 35% to 40% and are widely used as separators for lithium secondary batteries having negative electrodes of carbon.
However, these conventionally known separators are known to deposit lithium metal on graphite negative electrodes due to repeated charge and discharge cycles. Furthermore, the repetition of charge and discharge of a battery is known to cause the growth of dendrite lithium, resulting in a short circuit of the battery and this problem needs to be solved (Patent Document 1). In contrast, providing uniform pores in a separator in a regular manner is known to have an effect of improving the electrical characteristics of a battery (Non-Patent Document 1).
Separately, it has been tried to use a polyimide having a high heat-resistance and high safety in the separator (Patent Documents 2 and 3). However, the pores formed in the conventional polyimide film have insufficient uniformity and density.    Patent Document 1: Japanese Unexamined Patent Application (Translation of PCT Application), Publication No. 2010-537387    Patent Document 2: Japanese Unexamined Patent Application, Publication No. 2011-111470    Patent Document 3: Japanese Unexamined Patent Application, Publication No. 2012-107144    Non-Patent Document 1: Kazuhei Miyahara, and three others, “Evaluation of fundamental properties of 3DOM PI separator and production of metal lithium secondary battery”, The 53rd Battery Symposium in Japan, proceedings, 3D21, p. 267 (2012)